The present invention relates generally to cathode-ray tube display devices. More particularly, the invention relates to a cathode-ray tube display device in which display data can be written at any desired position on the display screen of the device and in which characters, figures or the like displayed on the screen can be edited. Yet more specifically, the invention relates to a cathode-ray tube display device in which scanning lines are divided into a number of picture elements, the brightness of each picture element is controlled, and the image of character, figures or the like is formed and displayed on the screen utilizing a plurality of picture element matrices arranged in vertical and horizontal directions.
In general, in a display device of this type, data stored in a refresh or pattern memory at addresses corresponding respectively to picture elements on the screen of a cathode-ray tube is read out from the memory in synchronization with the raster scanning of the cathode-ray tube thereby to display the entire frame of data stored in the memory. Data displayed on the screen is written into or erased from the memory by an input keyboard or with an optical detector called "a light pen" which can be manipulated directy on the screen.
The invention provides a cathode-ray tube display device in which data corresponding to each minute picture element can be written or erased with a light pen.
Heretofore, in writing data on the screen or in erasing data from the memory and hence screen using a light pen, raster light is detected by the light pen and, in accordance with the detection output of the light pen, the count value of an address counter refreshing the data on the screen is latched to determine the position of the light pen on the screen. The count value of the address counter corresponds to the picture element on the screen. If it is desired that the data on the screen be capable of being written or erased with the light pen with a resolution of a single picture element, the size of the raster light within the detection range of the light pen must be sufficiently small that the position indicated by the light pen can be correspondingly accurately determined.
Furthermore, in order to determine the position of the light pen with high accuracy, it is necessary that the detection output of the light pen be maintained stable. For this purpose, it is required that the detection range of the light pen be relatively wide. Thus, the detection range of the light pen is generally made wide enough to cover several elements. Should the size of the picture elements be further reduced in order to increase the amount of data which can be displayed on the screen while the number of picture elements detected by the light pen is increased to stabilize the detection output, if the light pen is even slightly shifted, then the above-described count value will change.
As is clear from the above description, the previously used technique of writing and erasing data for every picture element is disadvantageous in that the detected position of the light pen is liable to shift. As the density of picture elements on the screen is increased and the size of the picture elements correspondingly decreased, the light pen must be operated or moved more accurately. However, in practice, an operator cannot hold the light pen sufficiently stable to maintain the desired accuracy requirements. Accordingly, as the size of picture elements is decreased in order to increase the amount of data displayed on the screen, difficulties in writing and erasing data with a resolution of a single picture element increase.
As an example, with reference to FIG. 1, where the data to be displayed on the screen is composed of 15 characters in each of 8 lines which are displayed by 248 dots (picture elements) arranged horizontally in each of 192 lines, each character is displayed in a dot matrix composed of 24 lines each having 16 horizontally arranged dots. When it is required to write a complicated character such as a Chinese character having thin lines in one of the blocks, the detected position of the light pen may have a high likelihood of being shifted depending on the angle at which the light pen is held. That is, it is rather difficult to write character data accurately.